This invention relates to removal of solid impurities from liquids. More particularly it relates to removal of contaminants from ink recirculating in an ink jet printer or the like.
In ink jet printing droplets of ink issuing from a print head nozzle, which is often an array of several nozzles, are selectively deposited on a recording medium. Ink not used for printing is removed from the print station through a gutter and fed back to an ink supply line for recirculation and reuse. In the short distance from nozzle to gutter, nonprinting ink often becomes contaminated with particles of dust, lint or even insect wings from surrounding areas. Because of close tolerances which must be observed in the ink jet nozzle apparatus, solid impurities must be removed from recirculating ink before it enters the return line to print head. Mechanical filters such as stainless steel screens have been used for this purpose as taught in U.S. Pat. No. 3,929,071 Cialcone, et al., "Ink Recirculating System for Ink Jet Printing Apparatus", and assigned to the same assignee as the present invention.
Ink jet line printers are provided with a plurality of nozzles and often pump gallons of ink per hour. Usually several levels of filtration occur in the recirculation system. Progressively finer filters are provided at each level. Heretofore, ink has been caused to flow through screens, plates, mesh, or membranes of various materials. It is at the first filter station that the largest particulate impurities are removed. Due to the nature of the impurities and the filter means used in the prior art, the printer must frequently be shut down to clean or replace clogged filters. Efficient throughput is thereby hampered. The problem, naturally, increases with the number of printing nozzles provided and the volume of ink pumped. For example, in an ink jet printer capable of producing a line of characters in less than a second, the volume of nonprinting ink, from a large number of nozzle arrays, e.g., 144, collected through a common gutter for recirculation through a common ink supply line is much greater than that recirculated in a character by character ink jet printer such as that disclosed in the above referenced U.S. Pat. No. 3,929,071. Because such a large volume of ink is recirculated in a short period of time in a line printer, filtering ink through a screen or mesh at the first filter station results in rapid filter clogging. The solution to frequent clogging suggested by the prior art involves merely increasing the effective filter area. Providing a larger filter area however, may lead to other problems related to physical size or cost constraints of the machine. There is therefore a limit to the size of filter which can be used.
It has been shown that large quantities of fluid can be nebulized, i.e., formed into an aerosol, in a short time by ultrasonic techniques. For example, typically the rates for water are 0.2cc/sec. at 60 watts at 1.3MHz. Higher rates can easily be achieved with a higher level of ultrasonic power from one or more transducers. The drop diameter at 1.3MHz is about 2 microns. During nebulization larger droplets may form, but gravitational forces cause them to fall back. The aerosol comprises smaller, lighter droplets. Ultrasonic nebulizers capable of producing such an aerosol are commercially available and are frequently used in the medical field for inhalation therapy involving a variety of medicaments.
Ultrasonic nebulizers have also been used to form mists of a variety of ink types including magnetic ink for the selective deposition thereof to a surface for purposes of coating, printing, copying or the like. Such nebulizer applications are disclosed in the following patent and copending applications assigned to the same assignee as the present invention, U.S. Pat. No. 3,959,798 "Selective Wetting Using a Micromist of Particles", Hochberg, et al., Ser. No. 576,407, filed May 19, 1975 by Hochberg, et al., "Micromist Jet Printer", and Ser. No. 581,058, filed May 1975, by Hochberg, et al., "Method and Apparatus for Recording Information on a Recording Surface Through the Use of Non-Directed Particles."
These applications disclose ultrasonic nebulization of ink or other marking fluid to form a mist which is then directed toward the receiving medium. The mist so generated is caused to contact the receiving medium using a carrier gas only or in combination with electrostatic or magnetic deflection or attraction. Droplets not used for printing are collected from the print station and subsequently returned to the supply line for recirculation.
It is to be noted the prior art ultrasonic nebulizers referred to above have been used only to generate a mist of particles upon which only operations relating to directing, charging, deflecting or the like are carried out prior to wetting a receiving medium.